[riscv-port] RFR: 8278192: riscv: remove unnecessary instruct of DecodeNKlass in C2 [v2]

[Fei Yang]

On Sun, 5 Dec 2021 03:26:11 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

>> There are two instructs for DecodeNKlass in C2, and the difference is only that which temporary register operand is used by them, xheapbase, t0 or t1. It's too complicated and the effect is almost invisible. So we just simplify the pattern by using a temporary register from the register allocation.
>
> Yadong Wang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8278192: riscv: remove unnecessary instruct of DecodeNKlass in C2 (2)

Please add the testing carried out for this change in the PR description part. Thanks.

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PR: https://git.openjdk.java.net/riscv-port/pull/22